Supposedly, Iconia and Nelvana were in star systems inside the Romulan Neutral Zone, rather than on either side of it. Or at least Varley was adamant that Iconia was "in" the RNZ, and the Romulan presence there was as much a violation as the Starfleet one. And the same was true of the supposed base on Nelvana III, with "within" and "in" used to describe the location.

Even a Zone only one lightyear thick could of course have numerous star systems within its shell thickness by sheer chance if it were wide enough overall. But we don't know the overall dimensions of the RNZ - although we know that parts of it appear close to Earth while others appear very distant, suggesting at least a few hundred lightyears for the diameter of the barrier between Earth and Romulus.

And whether that barrier is a 2D disk or a 3D eggshell, we still don't know for sure...